Oxygen therapy is used to supplement the oxygen in the atmosphere in a variety of applications. Examples include:
(i) For people with damaged lung function—chronic obstructive pulmonary disease (COPD), emphysema or asthma.
(ii) For high altitudes, where the partial pressure of oxygen is too low for sustaining a person, e.g. the dropdown masks in aircraft, or devices used in high altitude climbing.
(iii) For general oxygen therapy, where an additional amount of oxygen has a therapeutic effect on the patient.
(iv) For use with a nebuliser, where it is an advantage to deliver only the amount of drug that can be absorbed by the patient.
Conventional devices for oxygen therapy give a constant flow, typically by metering the gas through a flow-meter or a fixed orifice. The oxygen is delivered to the patient typically via a nasal cannula—a tube connecting the outlet of the regulator, to the nostrils, or to a mask that covers the mouth and nose.
Oxygen conserving devices seek to improve on the conventional oxygen therapy devices by avoiding wastage of oxygen. An ideal oxygen conserving device would deliver gas for about half a second at the start of inhalation, then would not deliver any more until the start of the next inhalation. In this way only the gas that goes deep into the alveoli is consumed, and the oxygen that would otherwise be wasted (either delivered during exhalation, or just enter the air passages and trachea and be exhaled, not absorbed) and lost with the exhaled gas is instead conserved.
The advantages of conserving the oxygen in this way are well known, and include including making an oxygen cylinder or other supply last about three times as long, and increasing the travelling range of a person dependent on oxygen therapy. They may also reduce the number of cylinders a gas company has to deliver. There are also potential therapeutic benefits in comparison to constant flow devices, such as less drying of the nasal tissues.
A number of electronically controlled oxygen conserving devices exist on the market, which have the disadvantages of having to use batteries, temperature range limitations, and bulk—the designs tend to be difficult to incorporate into a pressure regulator as a single unit, and so end up being a separate unit, with the need for pressurised tubing between the regulator and the conserving device.
A fundamental problem that needs to be overcome in the design of a conserving device is that the resistance of the cannula to a typical therapy flow may be of the order of 100,000 Pa, whereas the pressure drop at the nose on inhalation may be typically 50 Pa. This means that once flow is started the pressure at the nose is too small to be detected at the device if a standard single tube cannula is used. It also means that transducers within the device capable of reading the approx 10-20 Pa of pressure needed to detect the start of an inhalation would be damaged by the pressure during flow.
The standard way to overcome this limitation is to utilise a dual lumen cannula—one tube for delivering the flow to the user, and a separate tube for transmitting the pressure at the user's nose to a sensing point on the conserving device. However, dual lumen cannulas are unpopular, because they are less easily available than the single lumen type, require two connections to be made instead of one, and are more expensive to make. They also mean that the user is restricted to a specific cannula, and cannot use the cannula that is most comfortable for them.
A number of pneumatically operated oxygen conserving devices exist on the market. The simplest type, described for example in U.S. Pat. No. 5,360,000 operates like a digital demand valve—giving a constant flow during inhalation, and switching off during exhalation. These have the disadvantage that the gas that is delivered after the start of the breath is wasted in the breathing passages and never gets to the alveoli.
FIG. 1 of the accompanying drawings shows a pneumatically operated oxygen conserving device which combines together common features from known single-tube cannula devices. Note that FIG. 1 is not intended to show a particular known device in detail, but to represent the main features of known devices, as they pertain to the present invention.
The device receives an oxygen supply to an inlet 1. Typical known devices operate from a supply pressure obtained for example from the output of a medical pipeline system or regulator, or from the output of a medical regulator—at a pressure typically from 1 to 5 bar according to the country and application. They may also operate directly from a liquid oxygen delivery system, typically regulated to a pressure of 1.5 bar.
The device may be incorporated into a high pressure regulator that uses gas from a cylinder and reduces it to the operating pressure of the conserving device.
Gas entering at inlet 1 passes via an input line 10, through a control valve 2 and via an output line 11 to an outlet 12 for connection to the single-tube cannula (not shown). The control valve is controlled, as represented by a control line 13, by the level of pressure in a main control volume 14. When the pressure in the main control volume is high (at a point approaching the level of the supply pressure), the flow is off, and when the pressure in the main control volume is low, the flow is on.
The main control volume is pressurised from the input line 10 via a flow line 15 in which is placed a flow restrictor 16. The flow through the restrictor 16 is set such that the pressure build up in the main control volume 14 from a “flow on” condition to a “flow off” condition is the time for which flow is required—i.e. the amount of time from the start of a breath to give the ideal dose of oxygen.
A variable restrictor (flow adjuster) 17 is fitted in the output line 11a, 11b, dividing the line into two sections 11a and 11b respectively upstream and downstream of the restrictor. The restrictor 17 may alternatively be fitted in the input line 10. The purpose of restrictor 17 is to meter the amount of flow that is delivered during the “flow on” condition.
The device is triggered by negative pressure sensed in a sensing volume 18 connected via a sensing line 19 to the output line 11b. The level of pressure in the sensing volume 18 controls, as represented by the control line 20, a sensing valve 21, for example in the form of a diaphragm, which allows air from the main control volume 14 to vent, usually to atmosphere as illustrated, via a vent line 22. When the pressure in the main control volume drops to a sufficient level, the control valve 2 is opened to start flow to the patient. Immediately the control valve opens, the pressure in the sensing volume 18 rises, which closes the sensing valve 21 and stops the venting of the main control volume 14. From this moment, the pressure in the main control volume 14 goes up, fed from the input line 10 via the flow line 15 and restrictor 16, until the level of pressure in the main control volume 14 reaches a sufficient level to close the control valve 2 and cut off the flow to the outlet 12.
The fundamental problem now is that, as a result of the flow stopping, there is no longer an elevated pressure in the output line 11b to keep the sensing valve 21 closed. Therefore, if at this moment the patient is still inhaling, the sensing valve opens again, and the main control volume 14 vents, thus opening the control valve 2 again to deliver another pulse of oxygen. This second pulse of oxygen is likely to mainly go to waste, because it is not required, as discussed above.
These difficulties are recognised in the prior art, and various ways of overcoming them have been described. For example, in EP 1028770, the flow adjuster is provided just after the gas inlet and a reservoir volume is provided between this and the main control valve. The control valve flows are set smaller than the flow required, and the pressure in the volume builds up during exhalation, and gets delivered at the start of inhalation. The amount of gas delivered on subsequent pulses and wasted is therefore less than the amount that would be delivered without the reservoir in place. However, there is still significant wastage, because the second and subsequent pulses contain a significant amount of gas.
In U.S. Pat. No. 6,484,721, use is made of a tail of gas flow after the initial pulse in order to prevent the occurrence of a second pulse during the same inhalation period. However, in order to be effective, a not insignificant amount of gas has to be used, which is wasted. The end of the tail of flow is undefined, so does not give a clear end point where sensing is definitely on or off, but depends on the level of the patient's breath.